WATERFOWL MANAGEMENT HANDBOOK 13.3.3. Aquatic Invertebrates Important for Waterfowl Production Jan Eldridge that wetland. For example, invertebrates such as Bell Museum of Natural History leeches, earthworms, zooplankton, amphipods, University of Minnesota isopods, and gastropods are dependent on passive Minneapolis, MN 55455 dispersal (they can’t leave the wetland under their own power). As a result, they have elaborate mecha- Aquatic invertebrates play a critical role in the nisms to deal with drought and freezing. A second diet of female ducks during the breeding season. group that includes some beetles and most midges Most waterfowl hens shift from a winter diet of can withstand drought and freezing but requires seeds and plant material to a spring diet of mainly water to lay eggs in spring. A third group that in- invertebrates. The purpose of this chapter is to give cludes dragonflies, mosquitoes, and phantom managers a quick reference to the important inver- midges lays eggs in the moist mud of drying wet- tebrate groups that prairie-nesting ducks consume. lands during summer. A fourth group that includes Waterfowl species depend differentially on the most aquatic bugs and some beetles cannot cope various groups of invertebrates present in prairie with drying and freezing, so,they leave shallow wet- wetlands, but a few generalizations are possible. lands to overwinter in larger bodies of water. Man- agers can use the presence of these invertebrates to Snails, crustaceans, and insects are important inver- determine the effectiveness of water management tebrate groups for reproducing ducks (Table). Most regimes designed for waterfowl production. species of laying hens rely on calcium from snail The following descriptions of invertebrate natu- shells for egg production. The northern shoveler ral history are based on Pennak (1978). and gadwall are dependent on crustaceans that swim in the water and forage on algae and fine or- ganic matter. The northern shoveler has an en- larged bill and finely developed lamellae for sieving Invertebrate Natural History crustacea from the water. Early-nesting species such as northern pintails and mallards consume early-emerging midge larvae in addition to earth- OLIGOCHAETA (Aquatic and Terrestrial worms, which are often the most available food in Earthworms) ephemeral wetlands shortly after the snowmelt. Natural History: Earthworms mix the substrate The diving ducks consume free swimming am- soils and consume algae and detritus. Their distri- phipods or larger insects such as caddis fly and bution is usually not limited by temperature and dragonfly larvae that tend to occur in deeper water. many truly aquatic forms survive in low oxygen The community of invertebrates present in a concentrations. Some earthworms form cysts or co- wetland can indicate the history of water changes in coons that are transported by birds or the wind. Fish and Wildlife Leaflet 13.3.3. • 1990 1 Table. Invertebrate classification. The following is a Crustacea list of the taxonomy of aquatic organisms that will serve most management purposes. ANOSTRACA (Fairy Shrimp) Phylum Class Order General Description: Fairy shrimp gener- Annelida Oligochaeta ally swim on their backs. They have 2 (terrestrial stalked, compound eyes, 11 pairs of swim- and aquatic ming legs that resemble paddles, and no earthworms) hard external covering. Hirudinea Natural History: Fairy shrimp are com- (leeches) mon in small ephemeral and temporary Arthropoda Crustacea Anostraca (fairy shrimp) ponds early in spring. They glide upside down, beat- Conchostraca (clam ing their legs in a wave-like pattern from tail to shrimp) head. Their leg action draws food into the ventral Cladocera (water fleas) Copepoda(copepods) groove toward the mouth. They feed on algae, bacte- Ostracoda (seed ria, protozoa, and bits of detritus. shrimp) Fairy shrimp lay two kinds of eggs: summer Amphipoda (scuds eggs that hatch soon after laying, and resting eggs and side- that sink to the bottom, where they withstand dry- swimmers) ing or freezing and hatch the next spring. Larvae de- Insecta Ephemeroptera velop through a series of "nauplius" instars and ma- (mayflies) ture rapidly; some become adults in as few as 15 days. Odonata Importance to Waterfowl: Because fairy shrimp (dragonflies) Hemiptera (true bugs) are among the first invertebrates in spring, they Trichoptera (caddis flies) are consumed by early laying northern pintail and Coleoptera (beetles) mallard hens. They also occur in the diets of north- Diptera (flies and ern shoveler and blue-winged teal. midges) Lepidoptera (butterflies CONCHOSTRACA (Clam Shrimp) and moths) Mollusca Gastropoda General Description: This organism is (Snails) enclosed in a shell-like outer carapace, and resembles a tiny swimming clam. Clam shrimp have 10−32 pairs of legs and 2 pairs of antennae. Natural History: Clam shrimp seem to Importance to Waterfowl: Terrestrial earth- prefer brackish water and swim by moving their worms in temporarily flooded, ephemeral ponds large biramous antennae in a rowing motion. Their early in spring are particularly important to early- natural history is similar to that of the fairy shrimp. nesting mallard and northern pintail hens. Importance to Waterfowl: Clam shrimp form an important part of the diet of laying gadwall hens, HIRUDINEA (Leeches) and also occur in the diet of mallards and northern Natural History: Some leeches are blood sucking shovelers. and forage on birds, mammals, fish, snails, insects, and earthworms. Leeches prefer warm water, and CLADOCERA (Water Fleas) are common in protected shallows. They are pri- General Description: Water fleas range marily nocturnal and require a substrate of rocks in size from 0.2 to 3.0 mm long. Superfi- or vegetation, so they are uncommon in wetlands cially, the body appears bivalve with the that have pure mud or clay bottoms. Leeches sur- abdomen and thoracic regions covered by vive winter and droughts by burrowing into the a carapace. The head is compact with two mud and becoming dormant. large, compound eyes. Water fleas have large anten- Importance to Waterfowl: Leeches are not par- nae with two segmented rami extending from a ticularly important to waterfowl as food, although large base. They have five to six pairs of biramous they are eaten by mallards in small amounts. legs that are hidden in the carapace. 2 Fish and Wildlife Leaflet 13.3.3. • 1990 Natural History: Water fleas use their antennae to OSTRACODA (Seed Shrimp) swim and appear to hop uncertainly in the water. General Description: Superficially, os- Their legs produce a current between the valves of tracods resemble tiny seeds. They are their carapace where food collects in the median usually less than 1 mm long with an groove and streams toward the mouth. Algae, detri- opaque, bivalve shell that varies in tus, and protozoans are the major items consumed. color. Water fleas migrate vertically, moving upward in the Natural History: Seed shrimp tolerate a wide evening and downward at dawn. They can exist in a range of environments, temperature, and water variety of temperature and oxygen concentrations. chemistry. Most species occur in water less than 1 Water fleas hatch from resting eggs at first m deep on varying substrates. Omnivorous scaven- thaw. As the water warms they reproduce rapidly, gers, they forage on bacteria, molds, algae, and often reaching a large population of 200−500 fleas fine detritus. Eggs can suspend development in per liter of water. The population wanes and by sum- dry and freezing conditions and some live as long mer, few are present in the ponds. Usually they re- as 20 years in the dried condition. produce parthenogenetically; however, as conditions Importance to Waterfowl: Seed shrimp, like co- deteriorate later in the season, they produce eggs. pepods, do not dominate the diet of laying females; Importance to Waterfowl: Water fleas form a ma- however, they are consumed in small amounts by jor part of the diet of the laying northern shoveler. gadwall, northern shoveler, and blue-winged teal. Cladocera are also consumed by gadwall and mal- lard hens. AMPHIPODA (Scuds, Side-swimmers, or COPEPODA (Copepods) Freshwater Shrimp) General Description: Most copepods General Description: Most am- − are less than 2.0 mm long. Usually phipods are 5 20 mm long with seg- they are drab in color; however, in mented thorax and abdomen. Their spring, some species are bright orange, eyes are usually well developed. purple, and red. The head and part of Natural History: Amphipods are primarily noctur- the thorax are fused in a cephalot- nal. They swim rapidly just above the substrate, rolling from side to back. Omnivorous scavengers, horax. The remainder of the thorax and abdomen they consume various plant and animal material. are segmented. Copepods have large antennae and They often browse on the film covering vegetation five thoracic segments that have legs that are used that is composed of microscopic plants, animals, for swimming. They have no abdominal appendages. and detritus. Natural History: Most copepods forage on algae, Amphipods are restricted to cold, shallow plankton, and detritus. Some forage by scraping water, and an abundance of oxygen is essential. food from the pond bottom and some by filtering They are generally found in permanent wetlands plankton from the water. Many swim in a smooth, where they can become abundant, and are not slow motion that is produced by the feeding move- generally adaptable to withstanding droughts. ments of the mouthparts and antennae, punctuated Importance to Waterfowl: Amphipods are very im- by jerky leg movements. The front antennae are portant to scaup, especially in fall, but they are not held stiff and act as a parachute to keep the cope- particularly important for dabbling ducks. Blue- pod from sinking. winged teal, gadwalls, and mallards consume small Copepods breed throughout summer, and are tol- amounts. erant of oxygen depleted water and adverse condi- tions such as drying and freezing. Some survive win- ter as resting eggs, some go into diapause on the Insecta wetland bottom and others form cysts or cocoons.